1. Field of the Invention
The present invention relates to a steel strip made of martensitic steel and to a production technique for a hoop for a continuously variable transmission of an automobile, which is obtained by using this steel strip, and specifically relates to a technology which produces the hoop for a continuously variable transmission with precise size and shape in a simple manner.
2. Description of the Related Art
A hoop for a continuously variable transmission of an automobile must have high-strength in use, and therefore the hoop is produced by using a high-strength material. In manufacturing the hoop, first, both ends of a steel strip, which is a flat material, are welded to each other to obtain an annular or cylindrical member. The member is then cut in a predetermined width. The member is subjected to ring rolling at a reduction of not less than 30% so as to remove irregularities of the welded portion and to provide a predetermined thickness.
As examples of a steel strip used for such a hoop for a continuously variable transmission, Japanese Patent Unexamined (KOKAI) Publication No. 59-80772 proposes to use a maraging steel. Japanese Patent Unexamined (KOKAI) Publication No. 2000-63998 proposes to use a high-strength metastable austenite stainless steel. Japanese Patent Unexamined (KOKAI) Publication No. 2001-172746 proposes to use a high-strength strain induced martensitic steel.
When the maraging steel is used, a high-strength hoop is obtained by rolling, performing a solution treatment for obtaining a uniform rolled structure, and aging. The solution treatment is performed at about 800° C. to 900° C. However, contraction and expansion with transformation in heating and cooling are repeated since the martensite reverse transformation point of the maraging steel is 600° C. to 800° C. Therefore, shape changes such as bending and swells in the width-direction may occur after the solution treatment even if the hoop is rolled in a precise size and shape.
Therefore, changes in size and shape cause by the solution treatment must be corrected in order to obtain precise size and shape as a hoop for a continuously variable transmission of automobile. As a method for obtaining a precise size and shape, for example, Japanese Patent Unexamined (KOKAI) Publication No. 11-173385 proposes to use a heat treatment, and Japanese Patent Unexamined (KOKAI) Publication No. 2001-105050 proposes to apply plastic deformation in cold working. However, considerable labor is required in the above-described method.
When the above described strong metastable austenite stainless steel is used, solution treatment after rolling is not required. Even if aging is required, it can be performed at a temperature of about 400° C. to 500° C., which is not more than the transformation temperature. Therefore, in this case, changes in the size and shape caused by the heat treatment are not significantly observed. Accordingly, a precise product can be obtained, if precise size and shape are obtained by rolling.
However, when conventional metastable austenite type steels described in Japanese Patent Unexamined (KOKAI) Publication No. 2000-63998 or Japanese Patent Unexamined (KOKAI) Publication No. 2001-172746 are used, work hardening in rolling is very large. Therefore, in this case, precise control of plate thickness or circumferential length are difficult, and precise size and shape when used as a hoop for a continuously variable transmission of automobile cannot be obtained.